This invention is directed to certain improvements in the construction and method of fabrication of natural tissue heart valves of the type characterized by having a cloth or fabric covered stent which incorporates a suture ring at its base so as to facilitate its implantation into the annulus or wall of the heart using conventional surgical procedures. The valve element itself is composed of pericardium which is mounted upon and sewn to the stent and, for example, in a semicircular heart valve is so configured as to define three cusps which will undergo opening and closing in response to reversals in the flow of blood through the annulus.
In the construction of natural tissue heart valves, it is important that the valve have a low profile and specifically that the tissue valve support posts on the stent be as short as possible so as to avoid rupture of the ventricular wall. Low profile porcine valves have been devised but have not been found to possess the optimum hydrodynamic characteristics desirable in a tissue valve. Bovine pericardium has been employed in the past as the tissue valve for enhanced hydrodynamic performance but has been placed along the interior of the stent in order to effect the necessary support of the valve. A greater effective orifice area can be achieved if the bovine pericardium can be supported around the exterior of the stent and fabric covering while avoiding the necessity of making the stent unduly rigid or inflexible; and further if the necessary stitching or anchoring of the tissue or pericardium be performed between the tissue and exterior of the stent so that the leaflets can open as wide as possible while leaving a smooth interior thereby achieving a lower pressure drop. Thus, a closely-related consideration in the construction of the valve is that the stitching employed between the cloth covering, stent, sewing ring and tissue be uniform or symmetrical throughout and in such a way as to avoid the introduction of increased bulk or non-uniformities in thickness at any point as well as to eliminate any exposed seams or fabric edges. Achievement of the foregoing will then permit utilization of a single piece of tissue as the valve element which can be securely mounted in place onto the cloth-covered stent while maintaining a uniform internal diameter when the tissue valve element is expanded to its open position.
Previously, it has been the practice to employ pressure fixation in the pre-forming of the tissue valve element. However, it is desirable to avoid pressure fixation so as not to affect the collagen bundles in the tissue and in general to provide for an improved method of fixation of the tissue valve element with respect to the cloth-covered stent.
Representative patents of interest in the fabrication and construction of natural tissue heart valves are U.S. Pat. Nos. 3,548,418, 3,983,581 and 4,035,849 to W. W. Angell et al; 4,084,268 to M. I. Ionescu et al; and 4,172,295 to R. J. Batten.